When Boeing Co. will attempt to launch its Starliner space taxi to the International Space Station remains unclear after its engineers detected a problem with valves on a propulsion system on the vehicle.

The launch of the aerospace giant’s Starliner capsule was scheduled for Aug. 3, but officials decided to postpone the flight because some valves on a Starliner propulsion system weren’t properly configured, according to the National Aeronautics and Space Administration.

Officials had said they could try to fly the vehicle to the space station on Aug. 4, but later nixed that possibility, saying engineers needed time to assess what happened with the valves.

Boeing has faced problems with the Starliner before. A botched effort in late 2019 dented the record of a company that has been at the forefront of U.S. space exploration, including the Apollo missions to the moon. The Starliner is the latest of an array of new rockets, capsules and other vehicles aimed at furthering U.S. ambitions in a new space race to the moon, Mars and beyond.

The Starliner would give the U.S. more options to reach low earth orbit and the space station. U.S. astronauts had to hitch rides on Russian Soyuz spacecraft to get there following the retirement of the Space Shuttle in 2011. NASA opted to outsource a replacement through its Commercial Crew Program and picked Boeing and Space Exploration Technologies Corp., the formal name for Elon Musk’s SpaceX, to provide space taxi services.

The CST-100 Starliner was slated to deliver more than 400 pounds of NASA cargo and crew supplies, and bring back material including oxygen tanks. A mannequin named Rosie the Rocketeer was also expected to be on board.

How can I watch the Starliner launch?

NASA plans to live stream the Starliner launch from the Cape Canaveral Space Force Station in Florida when the flight occurs.

NASA has said the Starliner mission is expected to run five to 10 days in all. The Starliner is expected to return to Earth at a facility in the White Sands Missile Range in New Mexico.

Wasn’t this supposed to launch on Tuesday?

The flight was postponed because of an issue with some valves in a vehicle’s propulsion system, the company and NASA said.

Boeing said its engineers conducting prelaunch checks of the Starliner initially detected the issue after electrical storms on Monday near the launch site.

Officials had scheduled the flight to begin July 30 but postponed it from that date after a Russian space vehicle mistakenly fired its thrusters while attached to the International Space Station the day earlier, forcing the facility into a tilt.

What went wrong with the 2019 mission?

The first uncrewed Starliner test flight in December 2019 failed to reach the space station. A software error left the spacecraft in the wrong orbit, ground controllers had difficulty communicating with the vehicle and another computer problem affected its thrusters. The spacecraft returned safely to Earth after two days, though NASA said it could have made it to the space station if astronauts were on board.

Boeing and NASA have completed multiple reviews and tests ahead of the latest launch. Working under a fixed-price contract with NASA, Boeing took a $410 million charge to pay for the second test mission, and will ferry the cargo to and from the space station for free.

The CST-100 Starliner is slated to deliver more than 400 pounds of NASA cargo and crew supplies.

The CST-100 Starliner is slated to deliver more than 400 pounds of NASA cargo and crew supplies.

Photo: joel kowsky/nasa/EPA/Shutterstock

When will the Starliner carry astronauts?

Before the Starliner can carry astronauts to the space station, Boeing needs to complete the test flight without crew members on board. A second Starliner capsule has been meant to carry astronauts to the space station as early as November, if the testing goes as planned and NASA certifies the Starliner. The reusable spacecraft is designed to fly 10 times and can be refurbished in six months.

How does it compare to the SpaceX Crew Dragon capsule?

The Starliner can carry seven astronauts or a mix of crew and cargo. It is launched on the same Atlas V series rockets developed by the joint venture between Boeing and Lockheed Martin Corp. that are used for military satellites. The spacecraft is autonomous, though there are backup controls that allow it to be piloted. It is designed for land-based returns, descending under parachutes, with air bags to cushion the landing.

The SpaceX Crew Dragon has carried cargo to the space station since 2012 and took its first astronauts to the space station in May 2020. The fully autonomous Dragon is launched on the SpaceX Falcon 9 rocket and has seven seats, though like the Starliner only four are used for NASA missions. It is designed to land under parachutes in the ocean.

Related Video

After years of cost overruns, errors and delays, Boeing’s space program is facing a major test: Later this year it will likely make its second attempt to launch its Starliner crew capsule to the International Space Station. WSJ looks at the company’s path to this crucial moment, and what’s riding on the test flight’s success. Illustration: Alex Kuzoian/WSJ The Wall Street Journal Interactive Edition

What else does Boeing do in space?

Boeing is one of the world’s largest space companies, with analysts estimating annual revenue of around $6 billion from the business last year. It has already helped refurbish the International Space Station this year and builds a range of satellites.

It is a pivotal year for Boeing’s space ambitions. Beyond the Starliner, it makes the main body of the Space Launch System, an enormous deep space rocket that will carry astronauts. After years of delay, a ground test in March paved the way for a potential launch by the end of the year as part of NASA’s Artemis moon exploration program.

Write to Micah Maidenberg at micah.maidenberg@wsj.com and Doug Cameron at doug.cameron@wsj.com